Gall midge pollination and ant-mediated fruit dispersal of Pinellia tripartita (Araceae)

The family Araceae consists of approximately 3,600 species that are characterized by cross-pollination using colorful pseudanthium and/or odor emission and endozoochory using glossy fruits. In contrast, a small deciduous aroid Pinellia tripartita possesses pale-green inflorescence without distinct smell and inconspicuous whitish-green fruits, suggesting that this species has a specialized reproductive system. However, no study has examined its reproductive biology in the field. To identify effective pollinators, we collected floral visitors from the introduced and five natural populations in the central and western Japan. We evaluated the selfing ability based on bagging experiments and the P/O ratio. Since myrmecochory was reported in some aroids with whitish fruits, we examined the ant-mediated removal of diaspores of P. tripartita, and two common herbs at the study site, Viola mandshurica (a myrmecochorious herb), and Oxalis dillenii. In all populations, gall midges were the dominant floral visitor, and pollen adhered to them. The fruit set rate significantly decreased in bagged inflorescences. The P/O ratio was intermediate between selfing and outcrossing plants. Ant-mediated diaspore removal was more frequent in P. tripartita than in the other two herbs. The decreased fruit set rate in bagging experiments and the intermediate P/O ratio suggested that P. tripartita conducts both cross- and self-pollination. Based on the floral visitor assemblage and ant-mediated fruit removal, P. tripartita appeared to employ tiny insects (gall midges and ants) for pollination and seed dispersal. Considering the similarity of reproductive traits, this tiny insects-mediated reproductive system may be common in the genus Pinellia.